Method for producing a multilayer film including at least one ultrathin layer of crystalline silicon, and devices obtained by means of said method

ABSTRACT

Method of fabricating a multilayer film having at least one ultrathin layer of crystalline silicon, the film being fabricated from a substrate having a crystalline structure and including a previously-cleaned surface. The method includes the steps of: a) exposing the cleaned surface to a radiofrequency plasma generated in a gaseous mixture of SiF4, hydrogen, and argon, so as to form an ultrathin layer of crystalline silicon having an interface sublayer in contact with the substrate and containing microcavities; b) depositing at least one layer of material on the ultrathin layer of crystalline silicon so as form a multilayer film, the multilayer film including at least one mechanically strong layer; and c) annealing the substrate covered in the multilayer film at a temperature higher than 400° C., thereby enabling the multilayer film to be separated from the substrate.

The invention relates to a method of fabricating a multilayer filmcomprising one or more ultrathin layers of crystalline silicon, the filmbeing fabricated from a substrate having a crystalline structure.

The multilayer film, based on crystalline silicon (c-Si), is fordepositing on a flexible or rigid support in order to fabricate solarcells or microelectronic devices.

The term “ultrathin layer” is used to mean a layer of thickness lying inthe range 0.1 micrometers (μm) to 5 μm.

Solar cells of high efficiency and microelectronic devices arefabricated using crystalline silicon.

The problem with solar cells and microelectronic devices based oncrystalline silicon is their high cost of fabrication due to the largequantities of silicon used in fabricating them.

Methods exist that seek to reduce the quantities of crystalline siliconneeded for fabricating solar cells and microelectronic devices.

More particularly, such methods enable heterojunction solar cells to beobtained that are fabricated on crystalline silicon substrates that arethin (50 μm to 100 μm) and that provide very high efficiencies (17% to22%).

Other methods also exist such as the method of cutting by implanting H⁺ions (the “smart cut” method), and the method of cutting by using ionsthat makes it possible to obtain films of crystalline silicon havingsmall thicknesses.

The smart cut method, as shown in FIG. 1, is known from the document byM. Bruel, “Separation of silicon wafers by the smart cut method”, Mat.Res. Innovat. (1999), 3, 9-13.

Those techniques implant H⁻ ions in crystalline silicon substrates atdoses lying in the range 10¹⁶ H⁺ ions per square centimeter (H⁺/cm²) to10¹⁷ H⁺/cm² in order to create defects (microcavities) at a certaindistance from the surface of the silicon substrate.

A substrate is obtained that has a layer of crystalline silicon that isvery thin (0.3 μm to 1 μm) and that has a large quantity of defects.

Thereafter, a hydrophilic bond is established with a second crystallinesilicon substrate. The second crystalline silicon substrate is put intocontact with the defect-containing surface of the first substrate. Heattreatment is performed at a temperature higher than 1000° C.

During the first stage of the heat treatment, the very thin film ofcrystalline silicon separates from the first substrate and bonds to thesecond substrate. During a second stage, the chemical bonds between thevery thin film of crystalline silicon and the second substrate areconsolidated.

The drawback of those methods is that they involve steps of implantingH⁺ ions and of performing heat treatment at high temperature (>1000°C.), which steps are complicated to implement and expensive. Hightemperature annealing also limits the methods to substrates made ofcrystalline silicon or refractory materials.

Furthermore, with those prior art methods, it is not possible to obtaincrystalline silicon films having a thickness of less than 0.3 μm orgreater than 1 μm, nor it is possible to make junctions or devicesdirectly.

Thus, an object of the invention is to propose a method of fabricating amultilayer film comprising at least one ultrathin layer of crystallinesilicon, which method is simpler, less burdensome, and enablescrystalline silicon films to be obtained that present thickness lying inthe range 0.1 μm to 5 μm.

To this end, the invention provides a method of fabricating a multilayerfilm comprising at least one ultrathin layer of crystalline silicon, thefilm being fabricated from a substrate having a crystalline structureand including a previously-cleaned surface.

According to the invention, the method comprises the following steps:

a) exposing said cleaned surface to a radiofrequency plasma generated ina gaseous mixture of SiF₄, hydrogen, and argon, the power density of theradiofrequency plasma lying in the range 100 milliwatts per squarecentimeter (mW/cm²) to 650 mW/cm², the pressure of the gaseous mixturein the range 200 pascals (Pa) to 400 Pa, the temperature of thesubstrate lying in the range 150° C. to 300° C., the flow rate of SiF₄lying in the range 1 cubic centimeter per minute (cm³/min) to 10cm³/min, the flow rate of hydrogen lying in the range 1 cm³/min to 60cm³/min, the flow rate of argon lying in the range 1 cm³/min to 80cm³/min, so as to form on said cleaned surface an ultrathin layer ofcrystalline silicon comprising a plurality of sublayers including aninterface sublayer in contact with the substrate and containingmicrocavities;

b) depositing at least one layer of material on said ultrathin layer ofcrystalline silicon so as to co-operate with said ultrathin layer ofcrystalline silicon to form a multilayer film, said multilayer filmincluding at least one mechanically strong layer so as to form amultilayer film having sufficient strength to enable said multilayerfilm to be separated without damaging the ultrathin layer of crystallinesilicon; and

c) annealing the substrate covered in said multilayer film at atemperature higher than 400° C., thereby enabling said multilayer filmto be separated from the substrate.

A self-supporting multilayer film is thus obtained. The multilayer filmmay then be transferred onto a mechanical support that is flexible orrigid, and not crystalline, and the crystalline substrate may be reused.

In various possible implementations, the present invention also relatesto the following characteristics, which may be considered in isolationor in any technically feasible combination, each of which provides itsown specific advantages:

the duration of step a) lies in the range 1 minute (min) to 5 hours (h);

during step a), the power density of the radiofrequency plasma is 500mW/cm², the pressure of the gaseous mixture is 293 Pa, and thetemperature of the substrate covered in said multilayer film is 200° C.;

during step a), the gaseous flow rate of SiF₄ is 3 cm³/min, the gaseousflow rate of hydrogen is 5 cm³/min, and the gaseous flow rate of argonis 80 cm³/min;

the mechanically strong layer is a layer of chromium deposited on one ofthe layers of the mechanically strong multilayer film by a vacuumevaporation method;

during step c) the substrate covered in said mechanically strongmultilayer film is heated up to 800° C.;

in step b), said layer of chromium is covered in a layer of polyamide,and in step c), the substrate covered in the multilayer film is heatedup to a temperature lying in the range 300° C. to 500° C.;

the surface of the substrate is initially covered in a layer of siliconoxide and the fabrication method includes, prior to step a), a step a′)of cleaning the surface of the substrate that is initially covered insilicon oxide;

said step a′) comprises an operation of exposing said surface to aradiofrequency plasma generated from a fluorine-containing gas, therebyetching the layer of silicon oxide, and an operation of exposing saidsurface to a radiofrequency plasma of hydrogen;

the steps a′) and a) are performed in the same reaction chamber of aplasma-enhanced chemical vapor deposition (PECVD) reactor;

step a′) comprises a standard cleaning operation using a wet methodbased on deionized water and HF;

step b) includes an operation of depositing a passivation layer (nitrideor silicon oxide) prior to depositing the mechanically strong layer;

in step b), the deposited material is crystalline silicon or germanium,which may be intrinsic or doped; and

during step b), a first layer of P- or N-doped material is deposited onsaid ultrathin crystalline silicon layer, said first layer of P- orN-doped material being covered in a second layer of N- or P-dopedmaterial in order to obtain a P-N or an N-P junction.

Thus, the invention provides a multilayer film fabrication method thatis more simple, less onerous, and more flexible, and that enables filmsof crystalline silicon to be obtained having thickness lying in therange 0.1 μm to 5 μm, while presenting a high degree of crystallinity.

It is no longer necessary to use a method of cutting by implanting H⁺ions, which method is complicated to implement and expensive. The methodof the invention is an alternative to that method of cutting.

The method of the invention is implemented in part in a PECVD reactor,unlike methods of the prior art (e.g. the “smart cut” method), therebyenabling the ultrathin layer of crystalline silicon to be deposited atlow temperature. The substrate is not consumed.

Furthermore, unlike prior art methods, the invention makes it possibleto use lower annealing temperatures (in the range approximately 400° C.to 600° C.) as compared with approximately 1000° C. in prior artmethods.

The invention also makes it possible to make stacks of doped andnon-doped layers so as to form P-N or N-P junctions. In prior artmethods, doping is determined by the substrate, which is limiting.

In the invention, it is also possible to incorporate other elements suchas germanium, and thus to make Si/Ge multilayer films, for example. Inthe prior art (“smart cut”) methods, it is not possible to depositmaterials other than the material of the substrate.

The invention is described in greater detail with reference to theaccompanying drawings, in which:

FIG. 1 shows a prior art method of cutting by implanting ions;

FIG. 2 shows a method of fabricating a multilayer film including anultrathin layer of crystalline silicon, in a first implementation of theinvention;

FIG. 3 shows a method of fabricating a multilayer film having anultrathin layer of crystalline silicon, in a second implementation ofthe invention;

FIG. 4 a is a graph plotting the deposition rate of the fine epitaxialcore sublayer of the ultrathin layer of silicon and the compositionthereof as a function of the flow rate of hydrogen;

FIG. 4 b is a graph plotting the thickness of the interface sublayer andthe composition thereof as a function of the hydrogen flow rate;

FIG. 4 c is a graph plotting the roughness of the surface sublayer andthe composition thereof as a function of the hydrogen flow rate; and

FIG. 5 is a detail view of the ultrathin layer of crystalline silicon.

FIG. 1 shows a prior art method of cutting by implanting ions.

The method comprises a first step 1) of implanting H⁺ ions in a firstsilicon substrate S1 that is covered in a thin layer of silicon oxide,at a dose rate lying in the range 10¹⁶ H⁺/cm² to 10¹⁷ H⁺/cm², in orderto create defects (microcavities) at a certain distance from the surfaceof the silicon substrate.

A substrate is obtained that includes a crystalline silicon layer 1 thatis very thin (thickness lying in the range 0.3 μm to 1 μm) and that isfull of defects.

Thereafter, a second step 2) is provided that consists in cleaning thefirst crystalline silicon substrate S1 and a second crystalline siliconsubstrate S2, and in establishing hydrophilic bonds with said secondcrystalline silicon substrate S2 at ambient temperature. The secondcrystalline silicon substrate S2 is put into contact with the surface ofthe first substrate S1 that has the defects.

A third step 3) of heat treatment is performed at a temperature higherthan 1000° C. During an initial stage of heat treatment (400° C. to 600°C.), the very thin crystalline silicon film 1 separates from the firstsubstrate S1 while remaining bonded to the second substrate S2. During asecond stage of heat treatment (T>1000° C.), the chemical bonds betweenthe very thin crystalline silicon film 1 and the second substrate S2 areconsolidated.

A fourth step 4) consists in polishing the surface of the very thincrystalline silicon film 1 on the second substrate S2.

The drawback of that method is that it involves steps of implanting H⁺ions, and of performing heat treatment at high temperature (higher than1000° C.), which steps are very complicated to implement and expensive.

Furthermore, with that prior art method, it is not possible to obtaincrystalline silicon film at thicknesses of less than 0.3 μm, nor morethan 1 μm.

FIG. 2 shows a method of fabricating a multilayer film 2′ including anultrathin layer of crystalline silicon 2 from a substrate S ofcrystalline structure, in a first implementation of the invention.

The substrate S has a surface that has previously been cleaned, i.e. asurface without oxide. The cleaning method is described below.

The substrate S may be a substrate of crystalline silicon or ofcrystalline germanium, for example.

The substrate S may be a <100> FZ, CZ, etc. substrate. It may bepolished on both faces, for example. It may have any resistivity. In theexamples below, it presents resistivity lying in the range 1ohm-centimeter (Ωcm) to 5 Ωcm.

The method of the invention may be applied to one or both opposite facesof the silicon substrate.

In the examples below, the substrate S is a substrate of crystallinesilicon.

The method of fabricating a multilayer film 2′ comprises a step a) ofexposing the cleaned surface of the substrate S to a radiofrequencyplasma generated in a gaseous mixture comprising SiF₄, hydrogen, andargon, so as to form on the surface of the substrate S an ultrathinlayer of crystalline silicon 2 including microcavities. Thesemicrocavities contain hydrogen.

The power of the radiofrequency plasma lies in the range 10 watts (W) ata power density of 100 mW/cm² to 60 W at a power density of 600 mW/cm².The pressure of the gaseous mixture lies in the range 200 Pa to 400 Pa.

The temperature of the substrate lies in the range 150° C. to 300° C.,the flow rate of SiF₄ lies in the range 1 cm³/min to 10 cm³/min, theflow rate of hydrogen lies in the range 1 cm³/min to 60 cm³/min, and theflow rate of argon lies in the range 1 cm³/min to 80 cm³/min.

The duration of the step a) preferably lies in the range 1 min to 5 h.It may also be less than that or greater than that. For example, whenstep a) is performed over 10 min, an ultrathin layer of crystallinesilicon 2 is obtained that presents a thickness of 0.15 μm.

The thickness of the ultrathin layer of crystalline silicon 2 depends onthe duration of step a) and on the flow rate of SiF₄.

For example, for a deposition speed of about 0.3 nanometers per second(nm/s), a duration of 10 min for step a) may correspond to an ultrathinlayer of crystalline silicon 2 having a thickness of 0.18 μm. A durationof 5 h for step a) may correspond to an ultrathin layer of crystallinesilicon having a thickness of 5.4 μm.

In preferred manner, during step a), the power of the radiofrequencyplasma is 50 W at a power density of 500 mW/cm², the pressure of thegaseous mixture is 293 Pa, the duration of the exposure to theradiofrequency plasma is 30 min (in order to deposit an ultrathin layerof crystalline silicon 2 having a thickness of 0.5 μm), and thetemperature of the substrate S is 200° C.

In still more preferred manner, the gaseous flow rate of SiF₄ is set at3 cm³/min, the gaseous flow rate of hydrogen lies in the range 1 cm³/minto 60 cm³/min, and the gaseous flow rate of argon is 80 cm³/min.

In still more preferred manner, the gaseous flow rate of SiF₄ is set at3 cm³/min, the gaseous flow rate of hydrogen is set at 5 cm³/min, andthe gaseous flow rate of argon is 80 cm³/min. This obtains an H₂/SiF₄ratio of 1.66.

After this step a) of exposing a surface of the substrate S to aradiofrequency plasma of H₂/SiF₄/Ar for a duration of 30 min under theabove optimum conditions, an ultrathin layer of crystalline silicon 2 isobtained that is made up of three sublayers comprising an interfacesublayer 19, an epitaxial core sublayer 20, and a surface sublayer 21.

The ultrathin layer of crystalline silicon 2 is shown in detail in FIG.5.

The interface sublayer 19 includes a large fraction of microcavities anda small fraction of crystalline silicon. This interface sublayer 19 isin direct contact with the surface of the substrate. It is positionedbetween the substrate S and the epitaxial core sublayer 20. It presentsthickness lying in the range 0 to 9 nm, which thickness depends on thehydrogen fraction used.

The epitaxial core sublayer 20 is made up of a monocrystalline siliconfraction, a fraction having large grains of crystalline silicon, and afraction having small grains of crystalline silicon. The epitaxial coresublayer 20 presents thickness lying in the range 90 nm to 170 nm (for10 min of plasma).

The surface sublayer 21 is made up of a fraction having large grains ofcrystalline silicon, a fraction having small grains of crystallinesilicon, and a fraction of SiO₂. It presents thickness lying in therange 0 to 5 nm.

The thicknesses of all of these sublayers depend on the hydrogenfraction used for diluting the SiF₄.

FIG. 4 a shows the deposition speed of the fine epitaxial core sublayer20 of the ultrathin silicon layer 2 and also the composition thereof asa function of the hydrogen flow rate (for flow rates in the range 1cubic centimeter per second (cm³/s) to 60 cm³/s).

The flow rate of SiF₄ is 3 cm³/s and the flow rate of Ar is 80 cm³/s inthe examples of FIGS. 4 a to 4 c.

Hydrogen flow rate is plotted in cm³/s along the abscissa axis 5. InFIG. 4 a, the left-hand ordinate axis 7 represents the deposition speedof the epitaxial core sublayer 20, and the right-hand ordinate axis 6represents the composition thereof (in percentage).

Curve 8 representing the deposition speed of the epitaxial core sublayer20 as a function of the hydrogen flow rate shows that at low H₂dilution, the deposition speed is low, lying in the range 0.1 nm/s to0.3 nm/s.

However, as the H₂ dilution increases, the deposition speed alsoincreases, with a maximum close to 0.3 nm/s (when the SiF₄ flow rate isequal to the H₂ flow rate, i.e. 3 cm³/s).

Any subsequent increase in the H₂ dilution has the consequence of alower deposition rate.

Curve 9 represents the fraction of monocrystalline silicon (c-Si) as afunction of the hydrogen flow rate.

Curve 10 represents the fraction of small grains of silicon as afunction of the hydrogen flow rate.

Curve 11 represents the fraction of large grains of silicon as afunction of the hydrogen flow rate.

The curves 9 to 11 show that the epitaxial core sublayer 20 has afraction of crystalline silicon that is large (about 80% to 95%), afraction of small grains of silicon lying in the range about 5% to 20%,and a fraction of large grains lying in the range about 1% to 5%. Thegreatest crystalline silicon fraction (approximately 95%) is obtainedfor a crystalline silicon film 2 deposited with identical flow rates ofSiF₄ and H₂ (3 cm³/min) and an argon flow rate of 80 cm³/min.

FIG. 4 b shows the thickness of the interface sublayer 19 and itscomposition as a function of the hydrogen flow rate (rates in the range1 cm³/min to 60 cm³/min).

Curve 12 represents the thickness of the interface sublayer 19 as afunction of the hydrogen flow rate.

Curve 13 represents the microcavity fraction as a function of thehydrogen flow rate.

Curve 14 represents the crystalline silicon fraction as a function ofthe hydrogen flow rate.

At a low H₂ flow rate (1 cm³/min), there is no interface sublayer 19 atall. As the H₂ flow rate increases, the interface sublayer 19 appearsand it reaches a thickness maximum at 10 cm³/min. An increase in the H₂flow rate gives rise to a decrease in the thickness of the interfacesublayer 19. The reason for such behavior is that at a low H₂ flow ratethe deposition speed is low and thus hydrogen can be desorbed from thethin film of silicon 2 that develops.

When the H₂ flow rate is greater, the deposition speed increases, andthe hydrogen is trapped in the interface with the crystalline siliconsubstrate. At an even higher H₂ flow rate, the deposition speeddecreases once more. A certain quantity of hydrogen is trapped in theinterface sublayer 19 and another portion is desorbed, having the resultof decreasing the thickness of the interface layer.

Curves 13 and 14 show that the interface sublayer 19 is made up mainlyof microcavities (about 80%) and a little crystalline silicon (about20%). Its composition is almost independent of the H₂ dilution.

FIG. 4 c shows the roughness of the surface sublayer 21 and itscomposition as a function of the hydrogen flow rate (flow rate in therange 1 cm³/min to 60 cm³/min).

Curve 15 represents the roughness of the surface sublayer 21 as afunction of the hydrogen flow rate.

Curve 16 represents the fraction of large grains of silicon as afunction of the hydrogen flow rate.

Curve 17 represents the fraction of small grains of silicon as afunction of the hydrogen flow rate.

Curve 18 represents the SiO₂ fraction as a function of the hydrogen flowrate.

Curve 15 shows that the roughness of the ultrathin layers of silicon 2increases with the H₂ flow rate from about 0.9 nm for a flow rate of 1cm³/min up to about 4.5 nm for a flow rate of 60 cm³/min.

From the curves 17 to 18, the SiO₂ fraction is almost independent of thehydrogen flow rate.

The interface sublayer 19 presents quantities of oxygen, hydrogen, andfluorine that are greater than those of the epitaxial core sublayer 20.This is due to the fact that the interface sublayer 19 presents manydefects (about 80% microcavities).

The method of fabricating a multilayer film 2′ also comprises a step b)of depositing at least one layer of material on the ultrathin layer ofcrystalline silicon 2 in order to cooperate with the ultrathin layer ofcrystalline silicon 2 to form a multilayer film 2′. The multilayer film2′ has at least one mechanically strong layer 3 so as to form amultilayer film 2′ that is mechanically strong and mechanically stable.

The mechanically strong layer 3 is the last layer that is deposited onthe multilayer film 2′.

The mechanically strong layer 3 is preferably a layer of metal. It maybe made of some other material such as glass or a polymer. The term“mechanically strong multilayer film 2′” is used to designate amultilayer film 2′ presenting sufficient strength to enable themultilayer film to be separated during a subsequent step as describedbelow without damaging the ultrathin layer of crystalline silicon 2.

The mechanically strong layer 3 is preferably made of chromium. It isdeposited on one of the layers of the multilayer film 2′ by a vacuumevaporation method. The thickness of the chromium layer 3 is greaterthan 100 nm. Chromium at a thickness of 150 nm ensures that themultilayer film 2′ has sufficient mechanical strength to enable itsubsequently to be separated from the substrate S.

In the embodiment of FIG. 2, during the step b), only one chromium layeris deposited on the ultrathin layer of crystalline silicon 2.

In another possible implementation, during deposition step b), one ormore epitaxial layers of semiconductor materials are deposited on theultrathin layer of crystalline silicon 2 in order to form a multilayerfilm 2′ having one or more layers of semiconductor materials between theultrathin layer of crystalline silicon 2 and the mechanically stronglayer 3.

The layers of semiconductor materials may be based on silicon,germanium, or SiGe, for example. They may be intrinsic, P-doped, orN-doped. The multilayer film 2′ may form a PIN or NIP, P-N, or N-Pjunction. It is possible to form junctions directly on the substrate S.

In a possible implementation, step b) includes an operation ofdepositing a passivation layer before depositing the mechanically stronglayer 3. This passivation layer may be a layer of silicon nitrideserving to obtain a low density of surface defects.

The method of fabricating a multilayer film 2′ also includes a step c)of annealing the substrate S covered in the multilayer film at atemperature that is higher than 400° C. and that preferably lies in therange 400° C. to 900° C., thereby enabling the multilayer film 2′ to beseparated from or peeled off the substrate S, and consequently enablingthe ultrathin layer of crystalline silicon 2 to be separated therefrom.

During step c), as the temperature is rising, the hydrogen atomsrecombine in the microcavities of the ultrathin layer of crystallinesilicon 2 so as to form microbubbles of H₂, thereby increasing thevolume of the microcavities in the ultrathin layer of crystallinesilicon 2.

In the implementation of FIG. 2, during the step c), the substrate Scovered in the multilayer film is heated up to 800° C. in an oven.

The temperature of the oven is then lowered to ambient temperature.

The oven may optionally be opened at about 250° C. in order to improvethe separation of the multilayer film 2′ from the surface of thesubstrate S.

In another implementation, as shown in FIG. 3, during step b), thechromium layer 3 is subsequently covered in a polyamide layer 4. Thesubstrate S covered in the multilayer film 2′ is then heated in an ovento a temperature in the range 250° C. to 350° C.

Thereafter, during step c), the substrate S covered in the multilayerfilm 2′ is heated up to a temperature in the range 300° C. to 500° C. inthe oven.

The temperature of the oven is then lowered down to ambient temperature.

The oven may optionally be opened at about 250° C. in order to improveseparation of the multilayer film 2′ from the surface of the substrateS.

In alternative manner, annealing step c) may be performed in a rapidthermal annealing (RTA) oven that has heater lamps enabling a very fasttemperature rise to be achieved. It is possible to reach 900° C. in afew minutes.

The method of fabricating a multilayer film 2′ may include, prior tostep a), a step a′) of cleaning the surface of the substrate that isinitially covered in silicon oxide (SiO₂), as described in document FR09/55766.

In a possible implementation, this step a′) comprises an operation ofexposing the surface to a radiofrequency plasma generated from afluorine-containing gas, thereby etching the layer of silicon oxide, andan operation of exposing the surface to a radiofrequency plasma ofhydrogen.

The fluorine-containing gas (or a gas based on fluorine) is preferablySiF₄ gas. Other fluorine-containing gases may be used, such as SF₆, forexample.

In another possible implementation, cleaning step a′) may be performedusing a wet method with a standard solution of hydrofluoric acid dilutedwith deionized water.

The advantage of using the RF plasma method is that steps a′) and a) canthen be performed in the same reaction chamber of a PECVD reactor,operating at a frequency of 13.56 megahertz (MHz).

The method of the invention may thus be implemented in a single PECVDchamber. The steps a′) and a) are performed in the same PECVD chamber,thus making it possible to avoid breaking the vacuum, to avoidcontaminating the substrate with external pollutants, to increase thespeed of the fabrication method, and to reduce fabrication costs.

This cleaning step a′) using a dry method is performed for a durationlying in the range 60 seconds (s) to 900 s. The power of the plasma liesin the range 1 W to 30 W, corresponding to a power density lying in therange 10 mW/cm² to 300 mW/cm². The pressure of the fluorine-containinggas lies in the range 1.33 Pa to 26.66 Pa.

This step a′) causes fluorine-containing elements to be fixed oradsorbed on the surface of the silicon substrate, thereby giving rise tosurface defects, in particular broken Si bonds.

These fluorine-containing elements are subsequently eliminated by theoperation of exposing the surface of the silicon substrate that includesfluorine-containing elements to a radiofrequency hydrogen plasma.

This operation of exposing the surface of the silicon substrate thatincludes fluorine-containing elements to a radiofrequency hydrogenplasma is performed for a duration lying in the range 5 s to 120 s, witha plasma at a power lying in the range 1 W to 30 W (power density lyingin the range 10 mW/cm² to 30 mW/cm²). The hydrogen pressure lies in therange 1.33 Pa to 133.32 Pa. This operation of exposure to aradiofrequency hydrogen plasma is optional.

The invention provides a method of fabricating ultrathin films ofcrystalline silicon that is simpler, less burdensome, and that makes itpossible to obtain crystalline silicon films having a thickness lying inthe range 0.1 μm to 5 μm with a high degree of crystallinity.

The invention claimed is:
 1. A method of fabricating a multilayer film(2′) comprising at least one ultrathin layer (2) of crystalline silicon,the film being fabricated from a substrate (S) having a crystallinestructure and including a previously-cleaned surface, the method beingcharacterized in that it comprises the following steps: a) exposing saidcleaned surface to a radiofrequency plasma generated in a gaseousmixture of SiF₄, hydrogen, and argon, the power density of theradiofrequency plasma lying in the range 100 mW/cm² to 600 mW/cm², thepressure of the gaseous mixture in the range 200 Pa to 400 Pa, thetemperature of the substrate (S) lying in the range 150° C. to 300° C.,the flow rate of SiF₄ lying in the range 1 cm³/min to 10 cm³/min, theflow rate of hydrogen lying in the range 1 cm³/min to 60 cm³/min, theflow rate of argon lying in the range 1 cm³/min to 80 cm³/min, so as toform on said cleaned surface an ultrathin layer of crystalline silicon(2) comprising a plurality of sublayers (19, 20, 21) including aninterface sublayer (19) in contact with the substrate (S) and containingmicrocavities; b) depositing at least one layer of material on saidultrathin layer of crystalline silicon (2) so as to co-operate with saidultrathin layer of crystalline silicon (2) to form a multilayer film(2′), said multilayer film including at least one mechanically stronglayer (3) so as to form a multilayer film (2′) having sufficientstrength to enable said multilayer film (2′) to be separated withoutdamaging the ultrathin layer of crystalline silicon (2); and c)annealing the substrate (S) covered in said multilayer film (2′) at atemperature higher than 400° C., thereby enabling said multilayer film(2′) to be separated from the substrate (S).
 2. A method of fabricatinga multilayer film (2′) according to claim 1, the method beingcharacterized in that during step a), the power density of theradiofrequency plasma is 500 mW/cm², the pressure of the gaseous mixtureis 293 Pa, and the temperature of the substrate (S) covered in saidmultilayer film (2′) is 200° C.
 3. A method of fabricating a multilayerfilm (2′) according to claim 1, characterized in that during step a),the gaseous flow rate of SiF₄ is 3 cm³/min, the gaseous flow rate ofhydrogen is 5 cm³/min, and the gaseous flow rate of argon is 80 cm³/min.4. A method of fabricating a multilayer film (2′) according to claim 1,characterized in that the mechanically strong layer (3) is a layer ofchromium deposited on one of the layers of the mechanically strongmultilayer film (2′) by a vacuum evaporation method.
 5. A method offabricating a multilayer film (2′) according to claim 4, characterizedin that during step c) the substrate (S) covered in said mechanicallystrong multilayer film (2′) is heated up to 800° C.
 6. A method offabricating a multilayer film (2′) according to claim 1, characterizedin that the surface of the substrate is initially covered in a layer ofsilicon oxide and the fabrication method includes, prior to step a), astep a′) of cleaning the surface of the substrate that is initiallycovered in silicon oxide.
 7. A method of fabricating a multilayer film(2′) according to claim 6, characterized in that said step a′) comprisesan operation of exposing said surface to a radiofrequency plasmagenerated from a fluorine-containing gas, thereby etching the layer ofsilicon oxide, and an operation of exposing said surface to aradiofrequency plasma of hydrogen.
 8. A method of fabricating amultilayer film (2′) according to claim 7, characterized in that thesteps a′) and a) are performed in the same reaction chamber of a PECVDreactor.
 9. A method of fabricating a multilayer film (2′) according toclaim 1, characterized in that step b) includes an operation ofdepositing a passivation layer prior to depositing the mechanicallystrong layer (3).
 10. A method of fabricating a multilayer film (2′)according to claim 1, characterized in that during step b), a firstlayer of P- or N-doped material is deposited on said ultrathincrystalline silicon layer (2), said first layer of P- or N-dopedmaterial being covered in a second layer of N- or P-doped material inorder to obtain a P-N or an N-P junction.
 11. A method of fabricating amultilayer film (2′) according to claim 2, characterized in that duringstep a), the gaseous flow rate of SiF₄ is 3 cm³/min, the gaseous flowrate of hydrogen is 5 cm³/min, and the gaseous flow rate of argon is 80cm³/min.
 12. A method of fabricating a multilayer film (2′) according toclaim 2, characterized in that the mechanically strong layer (3) is alayer of chromium deposited on one of the layers of the mechanicallystrong multilayer film (2′) by a vacuum evaporation method.
 13. A methodof fabricating a multilayer film (2′) according to claim 3,characterized in that the mechanically strong layer (3) is a layer ofchromium deposited on one of the layers of the mechanically strongmultilayer film (2′) by a vacuum evaporation method.